Rats initially fed a grain diet, then shifted to a beef diet, have shown significant increases in the activity of certain enzymes in the fecal bacterial flora, beta-glucuronidase, aryl-nitroreductase, and azoreductase. This increase in specific activity in these "Sentinel enzymes" occurred within 20 days of starting the meat diet. Advanced age also had an effect on these fecal enzymes. The meat-fed animals maintained their high levels throughout 20 months of life, with further increases in the levels of beta-glucuronidase. In grain-fed animals, the levels of fecal enzymes remained low during the first few months of life, but then rose to high levels after 12 months, and remained high. The carcinogen dimethylhydrazine caused an increase in beta-glucuronidase activity in both groups of animals. However, there was no effect on the levels of azoreductase or nitroreductase activity. Oral feeding of Lactobacillus acidophilus had no effect on enzyme levels in grain-fed animals. In meat-fed animals Lactobacilli lowered the activity of beta-glucuronidase and nitroreductase in the fecal flora. Our results indicate that diet, age, carcinogens, and Lactobacillus supplements can influence the metabolic activity of the bacterial flora.